Planetary collision likely source of Earth’s carbon

Illustration of planetary collision via A. Passwaters/Rice University, based on original courtesy of NASA/JPL-Caltech

The origin of carbon on Earth – one of the key building blocks of life on the planet – likely followed a collision between our planet and a Mercury-sized planet billions of years ago.

For billions of years, Earth’s carbon has been the source of life for almost every lifeform on the planet, but its origins have long perplexed researchers and geologists.

Based on current scientific understanding, our planet’s carbon should either be locked in the Earth’s core, or should have boiled away during Earth’s earliest formations. Yet neither is true.

Now, however, a new study published in Nature Geoscience by a Rice University team led by petrologist Rajdeep Dasgupta suggests that it came from elsewhere in the universe.

Based on the team’s ability to recreate the high-pressure and high-temperature conditions that exist deep inside Earth and other rocky planets, the likeliest scenario is that an embryonic planet of a similar size to Mercury smashed into Earth 4.4bn years ago.

With a sulphur or silicon-rich iron core, this embryonic planet would have melded with Earth’s own iron-rich core, resulting in carbon forming within the Earth’s mantle over billions of years.

A schematic depiction of early Earth’s merger with an embryonic planet similar to Mercury. Image via Rajdeep Dasgupta

Contradicts meteorite theory

This would differ from previous theories, which suggested that volatile elements like carbon, sulphur, nitrogen and hydrogen were formed by meteorites falling to Earth after the planet’s core finished forming.

By falling 100m years after the formation of the solar system, these newly-introduced elements would have survived being boiled away during the Earth’s formation.

“The problem with that idea is that, while it can account for the abundance of many of these elements, there are no known meteorites that would produce the ratio of volatile elements in the silicate portion of our planet,” said team member, Yuan Li.

By mapping out the concentrations of carbon that would arise from a collision with a planet whose core was enriched with sulphur or carbon, the team was able to reach this extraordinary, destructive conclusion.

“One scenario that explains the carbon-to-sulphur ratio and carbon abundance is that an embryonic planet like Mercury, which had already formed a silicon-rich core, collided with and was absorbed by Earth,” Dasgupta said. “Because it’s a massive body, the dynamics could work in a way that the core of that planet would go directly to the core of our planet, and the carbon-rich mantle would mix with Earth’s mantle.”